1. Field of the Invention
This invention relates to a CDMA radio multiplex transmitting device and a CDMA radio multiplex receiving device used for digital cellular communication or the like.
2. Description of the Related Art
A multiple access system is a line access system for plural stations to communicate in the same band at the same time, and one of the multiple access systems is a CDMA (Code Division Multiple Access) system. The CDMA system is a technology for multiple-accessing by spread spectrum spreading communication transmitting spectrum of an information signal spread into a fairly wide bandwidth comparing to the original information bandwidth. This is also called a Spread Spectrum Multiple Access (SSMA). A system multiplying a spreading sequence code as it is, by an information signal for the purpose of spread spectrum spreading is called a direct spreading CDMA system. In the direct spreading CDMA system, it is required to make strength of both an interference wave (communication wave from other station) and a desired wave identical at a receiving end in order that plural communications can jointly have the same frequency band. This is generally called as near and far problems. Solution of this far to near problem is a prerequisite for realization of a CDMA transmission system.
The near and far problems noticeably arise for the reception in a base station which receives radio waves from many stations (mobile stations, etc.) in different locations at the same time. Hence arises a necessity for mobile stations to execute a transmitter power control according to the condition of each transmission line.
Further, in radio communication system, a TDD (Time Division Duplex) system is known which tries to improve frequency utilization efficiency by using the same frequency band for both transmitting and receiving ends. The TDD system is also called as a ping-poing system which is a system for communicating by time-sharing the same radio frequency for both transmission and reception.
Still further, among detection systems in digital communication, a synchronous detection system has an excellent static characteristics compared to a delay detection system and it is a system where Eb/10 necessary for obtaining a certain average bit error rate (BER) is the lowest. As a system for compensating distortion of transmitting signals by fading, an insertion type synchronizing system is proposed (“a system for compensating phasing distortion of 16QAM for land mobile communication” by Mr. Seiichi Mihei, transactions of IEICE, B-11 Vol. J72-B-11 No. 1 pp. 7–15, 1989). In this system, a pilot symbols is periodically inserted into an information symbol to be transmitted and a detection is executed by inferring a transfer function of channel, that is to say, a line condition. Also, a system adopting the above described system to the direct spreading CDMA is proposed (“Characteristics of an insertion type synchronizing system in DS/CDMA” by Messrs. Azuma, Taguchi and Ohono, IEICE, technical report RCS94–98, 1994).
While, as a system making a synchronous detection possible in the direct spreading CDMA, there is a pilot channel. This is a system transmitting always independently from a channel which transmits the information data by making one channel (spreading code) as a reference signal for detection. An example of channel format is shown in FIG. 1. Synchronous detection of the information data is executed on the basis of an inferred phase by inferring a phase of a channel which transmits the information data from a signal of a inversely spread pilot channel. In this case, there are instances where a pilot signal (PL signal) is transmitted in a strong electric power to improve its reliability compared to a channel transmitting other information data, etc.
In the direct spreading CDMA, there is a multi code transmission which is a system where an information exceeding an information transmitting speed per one channel (one spreading code) is transmitted. This is a system where, by allotting to one user plural channels, that is to say, a plurality of spreading codes, the transmitting end multiplexes and transmits the information data by dividing it into plural channels. In the case where a synchronous detection is performed in the multi code transmission, it is supposed that said pilot symbols or pilot channel is used.
FIG. 2 shows an example of a channel format wherein the multicode transmission is performed by using the pilot symbols. The information data is transmitted by using N+1 channels (spreading codes 0˜N). A pilot symbols (pilot signal) 1201 is inserted into each channel at an interval of T period. Accordingly, it is possible that the receiving end executes a synchronous detection per each channel by using the pilot symbols.
However, in the above described conventional multi code transmission, a transmitter power of the pilot symbols is identical with the information data. Also, there is interference with the pilot symbols, particularly an effect of the interrelation among spreading codes, and for this reason it is difficult to perform a highly reliable synchronous detection.
While, FIG. 3 illustrates an example of a signal format in the case where the multi code transmission is performed by using the pilot channel. The receiving end executes a phase inference from a signal of a inversely spread pilot channel and, on the basis of the inferred phase, executes a synchronous detection of the information data in channels 1-N. However, in this case, since the PL signal is transmitted through the pilot channel in the information data transmitting period of channels 1-N, the information data signal is interfered with the PL signal. Also, the PL signal is interfered with the information data. Especially in the case where the PL signal is transmitted by an electric power higher than an information data transmitting channel so as to improve its reliability, a significant interference is caused.